1. Field of the Invention
The present invention relates to a switch supervisory device that is connected with an object section of an instruction path connecting a control unit for controlling a controlled object and a switch for setting the instruction path in a conductive state to send an input signal to the control unit and setting the instruction path in a non-conductive state. Further, the present invention relates to a control system having the control block, the switch and the switch supervisory device. Moreover, the present invention relates to a control method for controlling the control system.
2. Description of Related Art
An electronic control unit mounted on a vehicle has switches and a microcomputer. Each switch opens and closes a signal line through which an instruction is sent. The microcomputer controls a controlled object by sending the instruction through the signal line and each switch. This control unit has been disclosed in Published Japanese Patent Second Publication No. 3,711,849. Some of the switches used in the control unit are undesirably oxidized at contact points so as to lower the switching function. To prevent the deterioration of the switching function, it is required to remove oxidized components from the switches. To remove the oxidized components, it is required to supply electric current, set at a current strength larger than a predetermined value required to remove the oxidized components, to the switches during current-carrying periods. For example, the control unit is additionally provided with a current supply unit to supply a signal having a sufficiently large current to the switches. However, this current supply unit increases the manufacturing cost of the control unit.
Therefore, to prevent the oxidization of the switches, the control unit is generally designed such that a signal produced in an onboard battery set at a high voltage is sent to the switches. More specifically, a section of the signal line connecting the microcomputer with each switch is designed to be pulled up at a battery voltage of the onboard battery when the switch is in the off state. When the switch is turned on, a current path from the onboard battery to the ground through the switch is formed, and a signal produced in the battery is sent to the switch to remove oxidized components of the switch.
However, in this case of sending a signal produced in the onboard battery to each switch, the battery voltage (e.g., 12V) of the onboard battery is higher than the voltage (e.g., 5V) of a power source from which electric power is supplied to the microcomputer. Therefore, when the switch is set in the off state so as to disconnect the signal line from the ground, a signal produced in the onboard battery undesirably goes into the microcomputer through the signal line. Because this signal acts as a dark current, the control unit wastefully consumes electric power of the onboard battery.
To suppress this wasteful electric power, the control unit is, for example, structured so as to send a signal produced in the onboard battery to the signal line every predetermined period of time (i.e., every sending timing) and to check every sending timing whether or not the signal is sent to the microcomputer through the signal line as an input signal. However, in this case, it is required to check the sending of the input signal in synchronization with the sending of the signal produced in the onboard battery. Therefore, a constitutional component (e.g., a timer) for setting the sending timing so as to check the sending of the input signal in synchronization with the sending timing is undesirably required. This element increases the manufacturing cost of the control unit.